In view of present environmental regulations, the gasoline specification for sulfur content is becoming limited to lower levels. The main source of sulfur in gasoline is catalytic cracked naphtha, which can present typical values of 1,000 to 1,500 ppm wt, depending on FCC feedstock properties and operation conditions.
The conventional fixed bed hydrodesulfurization process (HDS) allows the attainment of low sulfur contents, but implies in the undesirable hydrogenation of olefins present in FCC naphtha, resulting in octane losses of the final gasoline pool.
Several selective hydrodesulfurization technologies have been developed, where selectivity means the ability to remove sulfur with minimum olefins hydrogenation.
At first, it was discovered that the composition of lower boiling point naphtha cuts showed lower sulfur and higher olefins content, while higher sulfur and lower olefins content were observed in heavier naphtha streams.
To take advantage of the olefins and sulfur distribution over the boiling point range, a technology was developed, which comprises splitting naphtha into a light and a heavy cut, promoting the desulfurization of the heavy cut, followed by the recombination of the light and the desulfurized naphtha. U.S. Pat. Nos. 3,957,625, 4,397,739 and 2,070,295 describe such a process.
Current HDS catalyst processes of olefinic naphtha feedstocks employ group VI transition metal oxides (MoO3 being preferred) and group VIII transition metal oxides (CoO being preferred), in sulfided form during operation conditions, deposited on a proper porous support.
More preferably, the acidity of said support may be lowered by the use of a metal additive, or present a low intrinsic acidity composition, as taught in U.S. Pat. Nos. 3,957,625, 4,334,982 and 6,126,814, which also consider different contents of selective metals as well as optimal metal ratio. Such catalyst properties favor HDS against the olefins hydrogenation function.
U.S. Pat. No. 2,793,170 suggests that lower pressures are favorable to a lower olefins hydrogenation, while not affecting the HDS reactions to the same extent. This patent also claims that, due to the recombination of H2S and mercaptans with the remaining olefins, reverse reactions, besides the sulfur removal reactions leading to H2S also occur, leading to formation of mercaptans (R—SH) and sulfides (R—S—R). Such reactions render difficult to accomplish lower sulfur contents without promoting at the same time olefins hydrogenation reactions to a critical extent.
Two-reactor process schemes with intermediate H2S removal are used to overcome the said recombination, as taught in U.S. Pat. No. 5,906,730.
Different two-reactor processes were also granted, where, in order to convert the mercaptans formed by recombination in the first reactor (U.S. Pat. No. 4,397,739), the second reactor is operated at a higher temperature.
Besides the process designs with more than one reactor, with or without intermediate fractionation, post-treatments are proposed in the literature, like the mercaptan sulfur extraction, see U.S. Pat. No. 6,228,254 and references cited therein.
In the main reactor of the two-reactor process, typical pressure range is of from 0.5 to 4.0 MPag, preferably of from 2.0 to 3.0 MPag. Temperatures in the range from 200° C. to 400° C. are considered, a preferred range extending from 260° C. to 340° C. The preferred space velocity (hourly processed volume per catalyst volume) or LHSV extends from 1 h−1 to 10 h−1. The hydrogen/feed ratio ranges of from 35 Nm3/m3 to 1800 Nm3/m3, with a preferred range being of from 180 Nm3/m3 to 720 Nm3/m3. The hydrogen purity is not usually claimed as an objective of the invention, being considered usually above 80%.
In spite of the numerous processes described in the art, there is a renewed interest in techniques for the sulfur removal of olefinic feedstocks. There are significant higher capital and operating expenses in a naphtha splitter, which can also limit the maximum sulfur removal, as some sulfur remains in the light naphtha.
Alternative processes have been proposed, as taught in U.S. Pat. No. 6,024,865 for the alkylation of thiophene sulfur to heavier compounds, which may lower the sulfur content of light naphtha.
Furthermore, the catalytic distillation of FCC naphtha is disclosed in U.S. Pat. No. 5,597,476, where diverse naphtha portions are subjected to different severity degrees.
In addition, reactive adsorption processes are considered in the current state-of-the-art technique.
The different process proposals demonstrate the relevance and difficulties inherent to the art of sulfur removal from olefin feedstocks. Therefore, the art still needs a HDS process able to reach maximum sulfur removal with minimum olefins hydrogenation, a result that can be attained according to the present invention by adding non-reactive diluent compounds to the hydrogen feed, such a process being claimed and described in the present invention.